the event is stated 1-20. it is as it was said when announced. you're not forced to do anything. you can lvl past 20 if you'd like.

if you qualified for any rewards during your lvls between 1 and 20 you will receive them after the event is over. note that once your past 20 you cannot increase your ranking it will only go down as others progress.

also know that this is the 1st event of many. so no need to get upset. there will be many more to come. this is an event for the main release of the game for all the new players considering some players had a head start and had events for them.